Diluted drift layer with variable stripe widths for power transistors

ABSTRACT

A multi-finger lateral high voltage transistors (MFLHVT) includes a substrate doped a first dopant type, a well doped a second dopant type, and a buried drift layer (BDL) doped first type having a diluted BDL portion (DBDL) including dilution stripes. A semiconductor surface doped the second type is on the BDL. Dielectric isolation regions have gaps defining a first active area in a first gap region (first MOAT) and a second active area in a second gap region (second MOAT). A drain includes drain fingers in the second MOAT interdigitated with source fingers in the first MOAT each doped second type. The DBDL is within a fingertip drift region associated drain fingertips and/or source fingertips between the first and second MOAT. A gate stack is on the semiconductor surface between source and drain. The dilution stripes have stripe widths that increase monotonically with a drift length at their respective positions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Nonprovisional patentapplication Ser. No. 14/671,572, filed Mar. 27, 2015, the contents ofwhich are herein incorporated by reference in its entirety.

FIELD

Disclosed embodiments relate to lateral high voltage metal oxidesemiconductor (MOS) power transistors having diluted drift layers,including LDMOS and DeMOS transistors.

BACKGROUND

Modern digital very-large-scale integration (VLSI) circuits commonlyoperate at supply voltages of around 2.5 volts or below. However,certain integrated circuits (ICs) call for additional on-chip circuitsoperating at higher voltages. Example circuits are input/output (IO)interface circuits with various off-chip system components such as powermanagement switches, analog input circuits conditioning transducersignals, or output analog drive functions for speakers or otheractuators.

One solution to this problem is to use multiple different gate oxidethicknesses and to build both low voltage transistors and high voltagetransistors on the same IC chip. This method increases processcomplexity and cost. An alternative solution is to use lateralasymmetric source and drain MOS transistors having a lightly dopedn-type gap between the drain and gate (for n-type devices) to enable useof higher drain to source voltages, such as laterally diffusedMetal-Oxide-Semiconductor (LDMOS) or drain-extended MOS (DeMOS) whichhave drain structures capable of operating at higher voltages ascompared to conventional symmetric MOS transistors.

In an LDMOS transistor a lightly doped lateral diffused drain region isconstructed between the heavily doped drain contact and the transistorchannel region. As the name implies, a lateral current is createdbetween drain and source. A depletion region forms in this lightly dopedlateral diffused region resulting in a voltage drop between the draincontact and the transistor gate. With proper design, sufficient voltagemay be dropped between the drain contact and the gate dielectric toallow a low gate voltage transistor to be used as a switch for the highvoltage.

Some lateral power transistors include “RESURF” regions which is shortfor reduced surface electric field regions. For purposes of this patentapplication, the term “RESURF” is understood to refer to a materialwhich reduces an electric field in an adjacent surface semiconductorregion. A RESURF region may be for example a buried semiconductor region(or layer) with an opposite conductivity type from the adjacentsemiconductor region (or layer). RESURF structures are described inAppels, et. al., “Thin Layer High Voltage Devices” Philips J, Res. 351-13, 1980. The RESURF region(s) for lateral power transistors aregenerally referred to as buried drift regions.

To raise the breakdown voltage of the lateral power transistor a dilutedburied drift layer may be used in the drift region at one end of thetransistor, which can be formed by a masked implant that enablesimplanting dilution stripes separated by masked (non-implanted) stripes.One or more high temperature annealing processes follow which results indopant from the implanted stripes diffusing into the non-implantedstripes which create more heavily doped stripes alternating with lessheavily doped stripes.

The DEMOS or LDMOS transistor can have a multi-finger layout with aplurality of source and drain fingers generally interdigitated with oneanother, or in racetrack layout which is in essence a single fingerdesign with an enclosed source or an enclosed drain. The diluted burieddrift layer for diluted buried drift layer designs generally sets thedrain-to-source breakdown voltage (BVDSS) for the lateral powertransistor, with the racetrack layout generally providing a higherbreakdown voltage that is near the ideal (planar) junction breakdownvoltage due to less junction curvature as compared to a lower BVDSS forthe multi-finger layout having higher curvature at the fingertipregions. Fingertip regions correspond to the curved distal end of thefingers that extend from linear (un-curved) regions of the fingertipAdvantages of multi-finger lateral power transistors (e.g., DEMOS orLDMOS transistors) include decreased parasitics effects and the abilityto change width (W), length (L), number of fingers, and number ofcontacts, which helps speed the transistor layout process.

SUMMARY

This Summary is provided to introduce a brief selection of disclosedconcepts in a simplified form that are further described below in theDetailed Description including the drawings provided. This Summary isnot intended to limit the claimed subject matter's scope.

Disclosed embodiments recognize for multi-finger lateral high voltagetransistors (MFLHVTs) including drain extended MOS (DEMOS) and laterallydiffused MOS (LDMOS) transistors having a conventional diluted burieddrift layer between the source and drain fingertips of the fingersreferred to herein as the fingertip drift region “FDR”, the use of afixed overdesign dimension (scaling, such as 200%) for the dilutionstripe width (e.g., at the drain end for n-channelMetal-Oxide-Semiconductor (NMOS)) of the FDR can limit thedrain-to-source breakdown voltage (BVDSS) of these transistors. Thisreduced BVDSS has been found to be due to the significant curvatureinduced electric field crowding present at the FDR, particularly at thehighest junction curvature portions, which has been verified by EmissionMicroscopy Imaging (EMMI) used to detect and localize certain integratedcircuit (IC) failures. For example, the BVDSS of a multi-finger LDMOScan be about 100V lower at about 700V as compared to the otherwiseequivalent racetrack version which has a BV close of about 800V which isthe ideal BVDSS due to a large endcap radius.

Disclosed embodiments provide a calculation (formula)-based dilutedburied drift layer (DBDL) design in the FDR for MFLHVTs which provides amonotonically increasing overdesign dimension (MIOD) for the dilutionstripe width along at least a portion of the DBDL which is within theFDR associated with source fingertips and/or drain fingertips. Thedilution stripe width corresponds to implanted buried drift layerregions. Disclosed DBDL designs have been found to improve the BVDSS ofsuch transistors by relieving electric field crowding in the highestjunction curvature portions of the FDR. Disclosed integrated circuits(ICs) can combine both n-type dilution for p-channel MOS (PMOS) MFLHVTsand p-type dilution for NMOS MFLHVTs.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, wherein:

FIG. 1A is a cross sectional view of example IC having an n-channelMFLHVT with an example DBDL portion and multiple horizontal currentchannels, and FIG. 1B is a top down view of the IC shown in FIG. 1A,according to an example embodiment.

FIG. 2A depicts a portion of a disclosed MFLHVT showing a source fingerhaving a source fingertip between a first drain finger and a seconddrain finger having a drain fingertip, respectively, according to anexample embodiment.

FIG. 2B is a top view depiction of the DBDL of a MFLHVT after BDLimplant having a MIOD dilution stripe width design, with the dilutionstripes corresponding to implanted regions with fifteen (15) exampledilution stripes designed based on disclosed scaling in the FDR,according to an example embodiment.

FIGS. 3A and 3B show a finger including a fingertip and a ½ of afingertip portion, respectively, used to show parameters used indisclosed DBDL designs, according to an example embodiment.

FIG. 4 shows comparative NLDMOS BVDSS data including data from adisclosed multi-finger NLDMOS device having a disclosed DBDL designhaving a MIOD such as shown in FIG. 2B, and a control racetrack NLDMOSdevice having a conventional DBDL having a fixed stripe overdesigndimension.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings,wherein like reference numerals are used to designate similar orequivalent elements. Illustrated ordering of acts or events should notbe considered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this disclosure.

Disclosed embodiments include MFLHVTs that have a DBDL design in the FDRhaving an MIOD, which has been found to improve the BVDSS by relievingelectric field crowding. Disclosed MFLHVTs also provide high current athigh voltage with reduced area by having multiple current channelsbetween the source and drain. Since current in disclosed MFLHVTs mayflow through multiple channels when disclosed transistors are turned ON,disclosed transistors provides high current with a reduced area. Themultiple current channels feature significantly reduces the arearequired for MFLHVTs including LDMOS or DeMOS transistors, therebysignificantly reducing cost.

The term “current channel” as used herein refers to a region of thesemiconductor substrate through which current flows. One current channelis isolated from another current channel by a diffusion of an oppositedopant type. One current channel may be shorted to the other currentchannel at the ends of the diffusion of opposite dopant type whichseparates the two current channels.

An example IC 150 having an n-channel MFLHVT (MFLHVT 100) with a p-typeBDL 132 including DBDL portion 132 a at the drain end near the commondrain 218 in the FDR adjacent to fingertips between interdigitatedsource and drain finger, with multiple horizontal current channels shownwith dashed lines as upper current channel 226 and lower current channel228 illustrated in a cross sectional view in FIG. 1A. Fingertip regionscorrespond to the curved distal end of the fingers that extend fromlinear (un-curved) regions of the fingertip. As noted above, a p-channelMFLHVT on the same IC such as IC 150 or another IC having n-typedilution may be realized by a reverse-tone device relative to MFLHVT100.

A top down view of the same is shown in FIG. 1B. Upper current channel226 is in the semiconductor surface 138 doped n-type between a topp-type surface layer 174 and BDL 132 which is p-type, while lowercurrent channel 228 is in an nwell 102 between the BDL 132 and thesubstrate 105. Substrate 105 is doped a first dopant type being p-typeand as noted above the semiconductor surface 138 is doped with thesecond dopant type being n-type. The vertical layer stack for MFLHVT hasa pnpnp structure thus providing 4 reduced surface electric field(RESURF) regions. However, because the top p-type surface layer 174shown in FIG. 1A is optional, the top p-type surface layer 174 can beremoved (skipped in the process) to provide a disclosed MFLHVT having apnpn structure thus having 3 RESURF regions.

The substrate 105 can comprise silicon, silicon-germanium, or othersemiconductor material. One particular arrangement is an epitaxialsilicon/germanium (SiGe) semiconductor surface on a silicon substrate105.

A symmetric S/D core logic PMOS transistor 50 has an nwell 146,source/drain diffusions 224 and transistor gate 202, and a symmetric S/Dcore logic NMOS transistor 60 has a p-type epi layer 130, source/draindiffusions 214 and the transistor gate 204 shown. The MFLHVT 100 asshown has two gates electrodes 206 and 208, and as noted above uppercurrent channel 226 and lower current channel 228, both between itscommon drain (drain) 218 and common source (source) 216. More than twohorizontal current paths may be provided if desired. The gate electrodes206 and 208 can comprises polysilicon, or alternatively metal.

When the gate including gate electrode 206 of the MFLHVT 100 is turnedON current flows through the upper current channel 226 between the topp-type surface layer 174 and the BDL 132. When the gate including thegate electrode 208 of the MFLHVT 100 is turned ON current flows throughthe upper current channel 226 and lower current channel 228. However,for power switching applications, the gate electrodes 206 and 208 may beshorted together to maximize the transistor ON-state current. Althoughtwo gates are shown in FIG. 1A, it is also possible to have a singlegate supporting both the upper current channel 226 and the lower currentchannel 228 so that disclosed MFLHVTs need only one gate.

There are dielectric isolation regions 162 at least partially in thesemiconductor surface shown as trench isolation (e.g., shallow trenchisolation (STI)) which can alternatively be field oxidation (FOX),including over the semiconductor surface 138, p-type epi layer 130 andnwell 146 of the IC 150 having gaps in the dielectric defining a firstactive area in a first dielectric gap region (hereafter source MOAT) 110where a common source 216 is formed and a second active area in a seconddielectric gap region (hereafter drain MOAT) 115 where the drain 218 isformed. The current channels 226, 228 are both shown sharing source 216and drain 218. The current channels 226, 228 are tapered and arenarrower and more lightly doped near the common drain 218 as compared totheir doping and width near the common source 216.

When gates including gate electrode 206 and 208 are both turned OFF andhigh voltage is applied to the common drain 218, an expanded depletionregion forms between the upper current channel 226 which is n-type andthe p-type surface layer 174 and BDL 132 and an expanded depletionregion forms between the lower current channel 228 which is n-type andthe BDL 132 and the substrate 105, so that upper current channel 226 andlower current channel 228 no longer provide a continuous current pathfrom the common source 216 to common drain 218. Sufficient voltage isdropped across these depletion regions so that the transistor gatesstacks including gate electrodes 206 and 208 may utilize the same lowvoltage gate dielectric as the logic transistors 50 and 60 to switch thehigh voltage.

FIG. 2A depicts a portion 200 of a disclosed MFLHVT showing aninterdigitated fingertip arrangement including a source finger 216 ahaving a source fingertip 216 a′ between a first drain finger 218 a anda second drain finger 218 b having a drain fingertips 218 a′ and 218 b′,respectively. The MFLHVT will have a plurality of repetitions of theinterdigitated finger arrangement shown. Associated with the sourcefingertip 216 a′ is the source FDR shown as 210 in FIG. 2A between anouter edge of the drain moat 115 and an outer edge of the source moat110 is the DBDL portion 132 a shown in FIG. 1A. Similarly, associatedwith drain fingertips 218 a′ and 219 b′ are respective drain FDRs 215between the outer edge of the drain moat 115 and the outer edge of thesource moat 110 which can also include a disclosed DBDL portion such asDBDL 132 a. The dilution in FIG. 2A heavier on the side near the drain218 equating to closer pitched dilution stripes with discernable stripesshown as compared to the degree of dilution on the side near the source216 which is less is shown in FIG. 2A by non-discernable dilutionstripes.

FIG. 2B is a top view depiction 250 of the source FDR 210 in FIG. 2Ashowing an example DBDL portion 132 a after BDL implant having a DBDLstripe widths corresponding to implanted regions which monotonicallyincrease in size as a function of the drift length (DL) at that theirlocation, according to an example embodiment. The fingertip center atthe beginning of the source fingertip 216 a′ is identified and shown as285, the linear drift region 290 is shown above the horizontal dashedline boundary shown as 295, and the source FDR 210 is below thehorizontal dashed line 295.

As used herein, DL is defined as the minimum spacing between the drainMOAT 115 and source MOAT 110, in the linear drift region being theshortest distance between source moat 110 and drain moat 115 (fromsource moat edge 110′ to drain moat edge 115′) which is a constant(shown as L), while in the source FDR 210 DL is defined as the distancebetween source moat edge 110′ and drain moat edge 115′ at the particularangle θ in the source FDR 210 (see FIGS. 3A and 3B showing θ). DBDLportion 132 a is shown including 15 example diluted DBDL stripes withstripes 132 a ₁, 132 a ₂ and 132 a ₃ identified in FIG. 2B that will beBDL implanted regions, alternating with non-implanted stripes 132 a′ ₁,132 a′ ₂ and 132 a′ ₃ are non-implanted regions which are masked duringthe BDL implant.

The parameter Lf is shown in FIG. 2B which is the over design dimensionfor the dilution stripes, where Lf is a fixed parameter (constant) for aconventional fixed layout design which determines the overall size ofthe source FDR 210, and L_(θ) is a variable in the FDR (see L_(θ) shownin FIG. 3B described below) that sets the stripe width. For theparticular layout shown in FIG. 2B, Lf is a fixed parameter while L_(θ)varies with θ ranging from 0 degrees to 90 or 180 degrees. The parameterL is also shown in FIG. 2B which is the distance from the edge of thesource MOAT 110 to center of the finger (see L shown in FIG. 3Bdescribed below). As described in the examples, for a n-channel MFLHVThaving a DBDL portion design such as shown in FIG. 2B, the BVDSSdifference between multi-finger and racetrack (single finger) layoutshas be found to be reduced from about 100 V to less than 40V.

When the MFLHVT comprises an NMOS device, for a constant distance fromthe fingertip center 285, the respective stripe widths of DBDLs 132 a inFIG. 2B can be seen to increase with an increasing angle θ relative tothe horizontal dashed line boundary 295 of the source FDR 210 with thelinear drift region 290, having a maximum width at 90 degrees. However,the stripe width change with θ is opposite for the FDR associated withthe drain fingertip such as the drain FDR 215 shown in FIG. 2A, wherethe respective stripe widths for the DBDL will decrease instead with anincreasing angle θ. This asymmetry reflects the disclosed dilutionconcept in the linear region, such as for an NMOS MFLHVT where thep-type DBDL 132 a is diluted more on drain side near drain 218 while thep-type DBDL 132 a is diluted less on the source side near the source 216as shown in FIG. 1A as described above.

Also for a fixed θ, as the distance from the fingertip center 285increases, the width of the DBDLs 132 a shown in FIG. 2B decreases,which as with the stripe width change with θ is opposite for the FDRassociated with the drain fingertip. However, some adjacent dilutionstripes can be the same stripe width due to a process limit, such as thestripes close to drain in linear drift region. Similarly the dilutionstripe spacing can be the same for some adjacent stripes due to aprocess limit, such as the dilution stripe spacing close to the sourcein the linear drift region. When the MFLHVT comprises a PMOS device, thedilution will be provided by an n-type buried layer and analogous to theNMOS device described above there will be less n-type buried layerdilution for the PMOS source side and more relative n-type buried layerdilution for the PMOS drain side.

Although NMOS MFLHVTs are generally described above, as noted above onehaving ordinary skill in the art can use the information disclosedherein for PMOS MFLHVTs by the n-doped regions being substituted byp-doping and vice versa. As used herein, when a diffused region isreferred to be doped with a particular dopant type, this means a regionin the semiconductor surface where doping concentration of the dopanttype referred to (e.g., n-type) is higher than the doping concentrationof dopants of the second type (e.g., p-type).

General aspects that make MFLHVTs including LDMOS/DEMOS transistors andprocessing to form the same can be found in a variety of references,including U.S. Pat. No. 8,470,675 entitled “thick gate oxide for LDMOSand DEMOS” to Sridhar et al., which is incorporated herein by reference.Briefly, a DEMOS transistor has an extended drain by adding a draindrift region between the drain and the channel of the device, trappingthe majority of the electric field in this region instead of the channelregion, and as used herein also includes a variant known as aDouble-Diffused Drain MOS (DDDMOS). A LDMOS transistor uses a draindrift region created by an extra doping similar to the DEMOS transistorstructure.

Regarding processing to form a disclosed DBDL, a DBDL layer mask isutilized having a plurality of stripes, such as to print a photoresistpattern with has exposed stripe shaped regions that be stripes.Implantation follows to form diluted buried drift layer stripes,followed by annealing. A drain comprising a plurality of drain fingersis formed interdigitated with a plurality of source fingers each dopedthe second dopant type. At least a first gate stack is formed on thesemiconductor surface between the source and drain.

Advantages of disclosed embodiments include relieving field crowding fortriple RESURF HV transistors which lack the top surface layer 174 orquadruple RESURF HV transistors such as MFLHVT 100 shown in FIG. 1A,without compromising the current contribution from the fingertip regionresulting from known removal of current conduction path(s). Anotheradvantage is disclosed DBDL designs are equation-based, enabling use forauto-design without the need for any human layout activity. Otheradvantages include low cost to implement since implementation onlyinvolves mask change(s), and there is no need for extra fabricationsteps.

Examples

Disclosed embodiments are further illustrated by the following specificExamples, which should not be construed as limiting the scope or contentof this Disclosure in any way. FIG. 3A shows a source or drain fingerincluding a linear finger portion 310 with an associated linear driftregion 290 and a fingertip 311 with an associated source FDR 210, with aBDL stripe 320 shown in the linear drift region 290, and FIG. 3B a ½ ofthe fingertip 311 shown in FIG. 3A now shown as 311 a and ½ of thesource FDR 210 now shown as 210 a, used to show parameters used indisclosed DBDL designs, according to an example embodiment. θ is shownin FIG. 3A and FIG. 3B and is the angle to the location of interest inthe FDR, and L_(θ) is shown in FIG. 3B which is the distance (length)from the outer edge of the drain MOAT 115 to fingertip center 285. rθnlshown in FIG. 3A is the trajectory of xnl within the source FDR 210 andxnl is one edge of the BDL stripe 320 in the linear drift region 290.rθnl lines up with xnl at θ=0. rs,d is the radius of the fingertip 311half circle depicted as a solid line as shown in FIG. 3A drawn from thefingertip center 285.

Example L_(θ) calculations within the FDR are now described. When θ<arctangent (arctg) (Lf/L):

L _(θ) =L/cos θ

and when π/2>θ>arctg (Lf/L):

$L_{\theta} = \frac{L\; {\sin \left( {{\arcsin \left( {\frac{L_{f}}{L}\cos \; \theta} \right)} + \left( {\frac{\pi}{2} - \theta} \right)} \right)}}{\cos \; \theta}$

An example drift length scaling equation is shown below:

$\frac{r_{\theta \; {nl}} - r_{s,d}}{x_{nl} - r_{s,d}} = {\left. \frac{L_{\theta} - r_{s,d}}{L - r_{s,d}}\Rightarrow r_{\theta \; {nl}} \right. = {r_{s,d} + {\left( \frac{L_{\theta} - r_{s,d}}{L - r_{s,d}} \right)\left( {x_{nl} - r_{s,d}} \right)}}}$

One then can solve for L_(θ), and then solve for rθ (shown as r_(θnl))from the above scaling equation, where r_(θ) defines how to draw(layout) the edge of each DBDL stripe in the FDR associated with thefingertips for source or drain fingers, such as source FDR 210 describedabove associated with the source fingertip. As L_(θ) increases, r_(θnl)increases which increases the width of the DBDL stripes. The width ofnon-implanted regions (gaps) between the DBDL stripes also increaseswith L_(θ). L=DL+rs,d; DL=L−rs,d is the constant drift length in thelinear drift region. L_(θ)=DL_(θ)+rs,d, DL_(θ)=L_(θ)−rs,d is the driftregion in the FDR.

In FIG. 2B which shows the case of a source FDR 210 for an NMOS device,each of the 15 DBDL stripes shown as DBDL 132 a within the source FDR210 has a width that varies with θ, specifically increasing in widthwith increasing _(θ) having a maximum width at 90 degrees, and beingsymmetric in its width pattern relative to the 90 degree line. Theequations above generate such a distribution since as θ increases L_(θ)increases, which increases r_(θnl) that increases the width of the DBDLstripes.

FIG. 4 shows comparative NLDMOS BVDSS data including the BVDSS from amulti-finger NLDMOS device having a disclosed DBDL having a variablestripe width overdesign dimension in the FDR such as shown in FIG. 2B,and a control racetrack NLDMOS device having a conventional DBDL havinga fixed stripe width overdesign dimension. The BVDSS difference betweenmulti-finger and racetrack (single finger) NLDMOS device layouts is seento be only about 40V, vs. in excess of over a 100 V reduction formulti-finger NLDMOS device having a DBDL design having a fixed stripeoverdesign dimension.

Disclosed embodiments can be used to form semiconductor die that may beintegrated into a variety of assembly flows to form a variety ofdifferent devices and related products. The semiconductor die mayinclude various elements therein and/or layers thereon, includingbarrier layers, dielectric layers, device structures, active elementsand passive elements including source regions, drain regions, bit lines,bases, emitters, collectors, conductive lines, conductive vias, etc.Moreover, the semiconductor die can be formed from a variety ofprocesses including bipolar, Insulated Gate Bipolar Transistor (IGBT),CMOS, BiCMOS and MEMS.

Those skilled in the art to which this disclosure relates willappreciate that many other embodiments and variations of embodiments arepossible within the scope of the claimed invention, and furtheradditions, deletions, substitutions and modifications may be made to thedescribed embodiments without departing from the scope of thisdisclosure.

1. A method of forming a lateral power MOS transistor, comprisingproviding a substrate doped with a first dopant type having a wellthereon doped a second dopant type with a semiconductor surface dopedsaid second dopant type on said well; forming dielectric isolationregions at least partially in said semiconductor surface having gapsdefining a first active area in a first dielectric gap region (firstMOAT) and a second active area in a second dielectric gap region (secondMOAT); forming a buried drift layer (BDL) doped said first dopant typehaving a diluted BDL portion (DBDL) including a plurality of dilutionstripes, comprising: forming a masking pattern using a diluted BDL maskhaving said plurality of dilution stripes with respective stripe widthsthat increase monotonically with a drift length at their respectivepositions, and implanting using said masking pattern; forming a draincomprising a plurality of drain fingers having drain fingertips in saidsecond MOAT interdigitated with a source comprising a plurality ofsource fingers having source fingertips in said first MOAT each dopedsaid second dopant type; wherein said DBDL is within a fingertip driftregion (FDR) associated with at least one of said drain fingertips(drain FDR) and said source fingertips (source FDR) between said firstMOAT and said second MOAT, and forming at least a first gate stack onsaid semiconductor surface between said source and said drain.
 2. Themethod of claim 1, wherein said respective stripe widths increase withan increasing angle θ relative to a boundary of said FDR with a lineardrift region having a maximum width at 90 degrees.
 3. The method ofclaim 1, wherein said FDR includes said source FDR and said drain FDR.4. The method of claim 1, further comprising forming a top surface layerin said semiconductor surface doped said first dopant type.
 5. Themethod of claim 1, wherein said first gate stack comprises a split-gateincluding a first gate stack and a second gate stack lateral to saidfirst gate stack.